US3866616A - Heart pacer - Google Patents

Abstract

A nuclear heart pacer having a heat-to-electricity converter including a solid-state thermoelectric unit embedded in rubber which is compressed to impress hydrostatic precompression on the unit. The converter and the radioactive heat source are enclosed in a container which includes the electrical circuit components for producing and controlling the pulses; the converter and components being embedded in rubber. The portions of the rubber in the converter and in the container through which heat flows between the radioactive primary source and the hot junction and between the cold junction and the wall of the container are of thermally conducting silicone rubber. The primary radioactive source material Pu238 is encapsuled in a refractory casing of WC-222 which in turn is encapsuled in a corrosion-resistant casing of platinum rhodium, a diffusion barrier separating the WC-222 and Pt-Rh casings. The Pt-Rh casing is in a closed basket of tantalum. The tantalum protects Pt-Rh from reacting with other materials during cremation of the host, if any. The casings and basket suppress the transmission of hard X-rays generated by the alpha particles from the Pu238. The outside casing of the pacer is typically of titanium but its surface is covered by an electrically insulating coating, typically EPOXY resin, except over a relatively limited area for effective electrical grounding to the body of the host. It is contemplated that the pacer will be inserted in the host with the exposed titanium engaging a non-muscular region of the body.

Description

United States Patent [191 Purdy et al.

[ HEART PACER [75] Inventors: David L. Purdy, Indiana; George J.

Magovern, Pittsburgh, both of Pa.; Nicholas Smyth, Bethesda, Md.

[73] Assignee: Coratomic lnc., Indiana, Pa.

[22] Filed: July 12, 1973 [21] Appl. No.: 378,636

OTHER PUBLICATIONS Myatl, Biomedical Engineering, Vol. 6, No. 5, May, 1971, p. 192-196.

Primary Examiner-William E. Kamm [57] ABSTRACT A nuclear heart pacer having a heat-to-electricity converter including a solid-state thermoelectric unit em- [45] Feb. 18, 1975 bedded in rubber which is compressed to impress hydrostatic precompression on the unit. The converter and the radioactive heat source are enclosed in a container which includes the electrical circuit components for producing and controlling the pulses; the converter and components being embedded in rubber. The portions of the rubber in the converter and in the container through which heat flows between the radioactive primary source and the hot junction and be tween the cold junction and the wall of the container are of thermally conducting silicone rubber.

The primary radioactive source material Pu is encapsuled in a refractory casing of WC222 which in turn is encapsuled in a corrosion-resistant casing of platinum rhodium, a diffusion barrier separating the WC-222 and Pt-Rh casings. The Pt-Rh casing is in a closed basket of tantalum. The tantalum protects Pt-Rh from reacting with other materials during cremation of the host, if any. The casings and basket suppress the transmission of hard X-rays generated by the alpha particles from the Pu The outside casing of the pacer is typically of titanium but its surface is covered by an electrically insulating coating, typically EPOXY resin, except over a relatively limited area for effective electrical grounding to the body of the host. It is contemplated that the pacer will be inserted in the host with the exposed titanium engaging a non-muscular region of the body.

5 Claims, 15 Drawing Figures PATENTEB FEB] 8M5 3 866 616sum 10F 7 FIG.5.

HEART PACER REFERENCE TO RELATED DOCUMENTS This application relates to an application, Ser. No. 378,513, to David L. Purdy for Electrical Generaton filed July 12, 1973 and assigned to CORATOMIC INC.

B CKGRQIJN OF THE N E O This invention relates to the generation of electricity by thermoelectric conversion of heat from a local primary source and has particular relationship to nuclear heart pacers or pacemakers. To the extent that this invention has other uses than in heart pacers it is understood that such uses are within the scope of this application.

A nuclear heart pacer includes a primary source of radioactive material, a thermoelectric converter which converts the heat from the source into electricity and an electrical circuit powered by the converter which converts the output of the thermoelectric converter into pulsations and controls the flow of the pulsations to the heart.

This prior-art heart pacer is encased in a conducting material such as titanium. In the use of this pacer muscle twitching has on occasions been experienced. In addition, the operation of this pacer has at times been irregular.

It is an object of this invention to overcome these disadvantages and to provide a heart pacer which shall not produce muscle twitching and shall operate with stability.

SUMMARY OF THE INVENTION This invention arises from the realization that the prior-art pacer, having a conducting casing throughout, makes electrical contact with muscles of the host in areas of the casing. The current flow through the surface of the casing then actuates these contacted muscles to twitch. In addition, these contacted muscles, during the normal activity of the host, inject electrical pulsations into the prior-art pacer which confuse its op eration and render this pacer at times unreliable.

The heart pacer in accordance with this invention is encased in a metallic container which is coated with an insulator, typically EPOXY resin, except over a limited area. The pacer is inserted into the body of the host with the conducting area out of contact with any muscular tissue of the heart.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a view in perspective of a heart pacer in ac cordance with this invention with parts of the outer container broken away to show the interior;

FIG. 2 is a view in end elevation with parts sectioned of the heart pacer shown in FIG. 1;

FIG. 3 is a view in section taken along lines III-III of FIG. 2;

FIG. 4 is a view in longitudinal section of the battery of the heart pacer shown in FIGS. 1-3;

FIG. 5 is a plan view of the strap assembly which applies pressure to compress the medium in which the thermoelectric unit is embedded;

FIG. 6 is a view in end elevation of the cylindrical bracket for positioning the radioactive block of the capsule shown in FIG. 4 as viewed from one end;

FIG. 7 is a view in end elevation of this bracket as viewed from the opposite end;

FIG. 8 is a view in section taken :along line VIIIVIII of FIG. 7;

FIG. 9 is a view in section taken along line IX-IX of the portion of FIG. 4 showing the thermoelectric unit;

FIG. 10 is a view in end elevation of the thermoelectric unit as viewed from the hot-junction end;

FIG. 11 is a view in end elevation of the thermoelectric unit as viewed from the cold-junction end;

FIG. 12 is a diagrammatic view showing the manner in which the individual thermoelectric elements of the thermoelectric unit are interconnected;

FIG. 13 is a block diagram of a typical electrical circuit of a heart pacer in accordance with this invention;

FIG. 14 is a view in perspective showing one way in which the heart pacer in accordance with this invention is installed in the host; and

FIG. 15 is a view in perspective showing another way in which the heart pacer in accordance with this invention is installed in the host.

DETAILED DESCRIPTION OF EMBODIMENT The apparatus shown in the drawings is a heart pacer 21 (FIGS. 1-3) including abattery 23, printedcircuit boards 25 and 27 (FIG. 2), asolidstate electronics package 29, astorage capacitor 31, amagnetic switch 33, atransformer 35 and anoutput assembly 37 for connecting the output of thetransformer 35 to the catheter orheart lead 39 which is placed on the heart muscle. Theboards 25 and 27 serve as a cradle for thebattery 23. Thebattery 23 and the circuit components 25-35 are embedded in apotting compound 41 of a resilient material in a container 43 (FIG. 3) typically of titanium. Thepotting compound 41 is typically predominately 2CN, a silicone rubber which is thermally insulating and which responds to pressure like a fluid, transmitting pressure uniformly in all directions. The 2CN is sold by Emerson-Cummings Corp. of Pittsburgh, Pa. However, aportion 47 of the compound between the cold-junction end 49 of thebattery 23 and thecontainer 43 is composed of a thermally highlyconducting material, typically ECCOSIL 4952, a silicone rubber. ECCOSIL 4952 is sold by Microtechtronics Corp., Buffalo, NY. Thecontainer 43 is encased in acoating 50 of EPOXY resin except for awindow 51. Thecontainer 43 serves as ground for the electrical circuit 23-35 and as a radio-frequency shield for the pacer and thewindow 51 serves to connect the ground to the body of the host. Thewindow 51 projects outwardly from the remainder of thecontainer 43 and is flush with the EPOXY coating 51) as shown in FIG. 2. Typically, theheart pacer 21 has an overall length of 2.45 inches, a width of 1.88 inches and a depth of 0.80 inch. Typically, the window ordimple 51 has a heighth of about 0.050 inch and is of oval shape about 1.8 inches by 1.1 inches. While the outwardly extending dimple is preferred, the conducting area could also be in a recession.

The battery 23 (FIG. 4) includes a primary source 61 and a solid-statethermoelectric converter 63. The source 61 is enclosed in a highly evacuatedcontainer 65 encompassed by aheat shield 67.

, dominately Pu Typically the source includes about 0.272 grams of Pu O This fuel is sufficient to operate the pacer according to this invention for 20 years without renewal of the source. This source delivers l 17 milliwatts initially and 100 milliwatts at the end of 20 years. The block 71 is mounted inpositioning bracket 73 ininner capsule 75, a hollow sphere. The bracket 73 (FIGS. 7, 8, 9) is formed of ahollow cylinder 77 havingtabs 79, spaced about 120, projecting outwardly from one end and tabs'81, interposed betweentabs 79 and spaced about 120, and projecting inwardly from both ends. The block 71 is supported within thebracket 73 by thetabs 81. The spacing between thetabs 81 is set to accommodate the length of the block 71 between thetabs 81. Typically,tabs 81 at one end are first projected inwardly; the block 71 is then inserted incylinder 77engaging tabs 81; thereafter,tabs 81 at the other end are bent to engage and hold the block 71. Thetabs 79 engage the inner surface ofsphere 75 and help to secure thebracket 73 in thesphere 75.Tabs 79 are bent inwardly from the 90 position about as shown in FIG. 4.

Theinner capsule 75 is formed ofhollow hemispheres 83 and 85, the rims of the hemispheres being provided withcooperative projections 87 and 89 respectively which are coextensively engaged to form thesphere 75. Thehemispheres 83 and 85 are closely dimensioned to about 0.001 inch. Typically, the sphere is composed ofa tantalum alloy, WC-222, has an inside radius of 0.125 inch and an outside radius of 0.155 inch. WC-222 has the following chemical composition: W 9.6-11.2%,1-1F 22-28%, C 0.008-0.0175%, Ta balance. Thebracket 73 typically has a length of about 0.186 inch and a diameter of about 0.125 inch. Theinner capsule 75 is fire resistance and has high strength so that theblock 77 remains locked in the inner capsule regardless of what impacts the heart pacer may suffer and also if the host should be cremated. The tantalum alloy also absorbs hard X-rays. Theinner capsule 75 is enclosed in anouter capsule 91 including a hood 93 formed of a hemisphere from which a cylinder extends; a spherical dish-shapedmember 95 is sealed to the rim of the cylinder. Typically, the hermisphere and cylinder of the hood 93 have an inner diameter of 0.316 inch and a thickness of about 0.010 inch. Themember 95 has the same thickness and is correspondingly dimensioned to sealto the hood 93. Theouter capsule 91 is compoxsed of platinum-rhodium alloy PtlORh. Theouter capsule 91 protects the source 61 against corrosion and oxidation. Adiffusion barrier 97, aluminum oxide, typically of about 0.001 inch thickness is disposed between theinner capsule 75 and theouter capsule 91. This barrier prevents the alloying of the inner andouter capsules 75 and 91.

During assembly the block 71 is secured in thebracket 73 with thetabs 81 holding the block. Thebracket 73 and block 71 are then inserted between thehemispheres 83, 85 and the hemispheres are joined at thesteps 87 and 89 into a sphere. The sphere is then welded in an inert-gas (argon) atmosphere. Thesphere 75 is then inserted between themembers 93 and 95 of theouter capsule 91 and this capsule is welded at the joint 99 between these members.

The unit 71-75-91 is disposed in a basket 101 typically of tantalum. The basket 101 is in the form of a hollow hemisphere terminating at its rim in a hollow cylinder. Typically, the basket 101 is about 0.005 inch in thickness. Near its outward end the basket 101 is welded to theflange 103 of a flanged sleeve 105, typically of tantalum. Thestem 107 of the sleeve 105 merges into theflange 103, at their inner surfaces, the transition surface being a spherical annular surface of the same radius as the dish-shapedmember 95. The dish-shaped member is seated in this surface. Thestem 107 is welded to adisc 109, typically of tantalum. The inner surface of thedisc 109 is spherical and coextensive with the spherical annular surface of the flange and of the same radius and themember 95 is seated in this surface. Thedisc 109 has an outwardly projecting rim 111.

A flanged disc 113 is brazed to the rim 111. This disc 113 is composed of the titanium alloy Ti6Al4V. Acylinder 115, also typically of Ti6Al4V, which contains theconverter 63 is welded to the flange 117 of the disc 113. The flange 117 is trepanned to limit the flow of heat from the weld. The alloy Ti6Al4V has high strength and low thermal conductivity and is used for this reason.

The evacuatedcontainer 65 is defined byouter sheel 121 and thecylinder 115. These members and 121 are joined by aring 123 typically of Ti6Al4V. Theshell 121 is typically composed of titanium and includes a hollowhemispherical end 125 from the rim of which a cylinder extends. A hollow frusto-conical shell 127 extends from the rim of the cylinder. The cylindrical rim of theend 125 and theshell 127 are joined by a weld. Abacking annulus 128, typically of titanium, is provided behind the welded joint. Theend 125 and theshell 127, throughout the major portion of lengths, have a thickness typically of 0.010 inch. But theshell 127 flares out at itsconstricted end 129, to a thickness of about 0.090 inch. Thethickened end 129 is chamfered on the outside and flattened on the inside and is welded to thering 123 around the flattened area. Thering 123 is internally welded to thecylinder 115. A reinforcingring 124 internally of thecylinder 115 forms a part of a welded joint between thecylinder 115 and thering 123. The ring is trepanned to suppress the flow of heat from the weld.

The vacuum withincontainer 65 is maintained by agetter 131 typically CERRALOY 400. Thegetter 131 is mounted in an annular space provided between thesleeve 107, theflange 103, and acylinder 133 which extends from the rim of the basket 101 and is bent inwardly atteeth 135 extending from its end. Thegetter 131 is held by a ring 137, typically of var-glass tubing.

Theheat shield 67 has ahemispherical section 141, acylindrical section 143 and aconical section 145. Thehemispherical section 141 includes a plurality ofhemispheres 147, typically of MONEL metal, concentric withsphere 75 and extending between the spherical part of the basket 101 and the inner surface of theend 125. The hemispheres have dimples 149 so that their spacing is maintained. Thecylindrical section 143 includes a tape of alternate layers ofMONEL metal foil 151, typically 0.125 inch wide by 0.001 inch thick, andE glass insulation 153, typically, 0.125 inch wide by 0.005 inch thick. The tape is wrapped about a center formed of the cylindrical parts of the hood 93 and the basket 101 and thestrip 133 which holds thegetter 131. Thecylindrical section 143 firmly engages the basket 101 and thestrip 133 and supports the asembly 717591, which is relatively heavy, radially and pre vents its displacement radially. Radial displacement of the assembly 7145-91 exerts a torque on the joint 115423-124 and may rupture this joint. So that thesection 143 firmly engages the assembly 71-7591,dimples 144 are pressed into theshell 125. Theconical section 145 includes a plurality of hollow frusto-conical shells 161 coaxial with thecylinder 115, typically composed of MONEL metal, each extending from thecylindrical shield 143 to a position above thering 123. Theshells 161 are bent over perpendicularly to thecylinder 115 near thering 123 and are engaged by anannular spacer 163, typically of Ti6Al4V.

Thehead 125 and theconical section 127 are initially separated pieces with thebacking ring 128 tack-welded to thehemispherical head 125. In assembling thecontainer 121 thering 123 is welded to theend 129 of theconical section 127 of thecontainer 65. The strip (typically 0.005 inch thick) which forms theholder 133 for thegetter 131 is wrapped around the cylindrical end of the basket 101. Thegetter 131 and the glass tubing 137 are inserted and theteeth 135 of the strip are bent so as to hold thegetter 131 and the tubing. The tape 151-153 is wrapped around the assembly 13310193. Thehemispherical shells 147 are positioned about the hemispherical portion of the basket 101 and thehead 125 is positioned over theoutermost shell 147. Th rims of theshells 147 engage thecylindrical shield 143. The frusto-conical shells 161 are stacked in thefrustoconical section 127 between thespacer 163 and the end of thesection 127. Several of thelarger shells 161a are of 0.005 inch thickness titanium; the other shells 161b are of 0.004 inch thick MONEL. Atube 171 of E-glass insulation is wound about eachshell 161 supporting the adjacent shell. Thesections 125 and 127 are then abutted withbacking ring 128 extending be tween the rims of these members (125, 127). The assembly is then placed in a chamber which is evacuated to a pressure of Torr and the joint between the rims of themembers 125 and 127 is welded and treated out so that the low pressure is maintained.

Thethermoelectric converter 63 is disposed incylinder 115. Theconverter 63 includes a solid-statethermoelectric unit 181 embedded or potted in a medium 183 which responds to compression like a fluid, hydrostatically, transmitting the compression in all directions.

Thethermoelectric unit 181 includes a plurality of positively and negatively dopedstrips 185 and 187, typically of bismuth telluride, embedded inpolymeric insulation 189 such as EPOXY. So embedded this block is itself stress resistant and protects thestrips 185 and 187 from rupture. Typically, there may be 81 ofsuch strips 185 and 187 arrayed as shown in FIG. 10. Successive negative and positive strips are interconnected by solder strips 191 (FIGS. 11, 12) at the hot-junction of theunit 181 and alternate pairs of positive and negative strips are interconnected by solder strips 193 (FIGS. 10, 12) at the cold-junction. Typically,positive strip 185a andnegative strip 187a are interconnected bysolder strip 191a at the hot-junction andnegative strip 187a andpositive strip 185!) are interconnected bysolder strip 193b at the cold-junction. Diagonally positioned positive and negative end strips 1185c (FIG. 11) and 187C are connected tooutput conductors 201 and 203. The pairs ofstrips 185 and 187 of the array of strips form thermocouples connected in series betweenconductor 201 andconductor 203.

The potting 183 includes a disc 211 (FIG. 41) ofthermally conducting resilient material, typically silicone rubber ECCOSIL 4952, interposed between the source 61 and thethermoelectric unit 181, ahollow cylinder 213 of thermally insulating material, typically rubber, SYLGARD 184, encircling theunit 181, and adisc 215 of ECCOSIL 4952 having an internally generally frusto-conical rearward projection. SYLGARD 184 is sold by Techtronic Corporation of Buffalo, NY. The potting compound 211-213-215 serves as axial support for the cylinder and prevents the assembly 71-7- 5-91-141-143 from buckling thecylinder 115.

Theconverter 63 is assembled in thecylinder 115. First, the cylinder 211 is deposited on the trepanned disc 113. Next, thethermoelectric unit 181 is positioned centrally on the disc 211. Then thecylinder 213 is deposited around theunit 181. A washer, 216, typically of titanium, is then positioned to cover thetrepan groove 217 of thedisc 123. Asplit ring 219, typically of Ti6A14V alloy is placed coaxially with thecylinder 115 on thewasher 216. Thering 219 is resilient but has a high restoring force. A cylindrical mass of thermally conducting material, typically ECCOSIL 4952, is then deposited in the cylindrical space defined by thering 219, thewasher 216 and thedisc 123. Before this mass is cured aplug 231 is positioned at the outer edge ofthe mass.

Theplug 231 is typically composed of electrolytic grade copper and has ahead 237 split or slotted at the center and abody 239 which terminates in a generally frusto-conical portion. Thebody 239 has a central opening which Communicates with anopening 241 in the head. The central opening has aceramic bushing 243.

Theconductors 201 and 203 are brought out centrally before the cylindrical mass is deposited within theinner ring 219, thewasher 216 and thedisc 123. After the mass is deposited, but before it cures, theconductors 201 and 203 are strung through theceramic bushing 243 of theplug 231 and brought out through the slot between the parts of thehead 237. Theplug 231 is then set at the outer end of the mass of thermally conducting material, while being maintained coaxial with thecylinder 115. As the mass is cured thering 219 prevents the mass from expanding radially. Once the mass is cured theplug 231 is pressed into the mass building up hydrostatic pressure within thepotting material 183. Thesplit ring 219 confines the mass radially but the gaps in this spring open to a predetermined spacing which measures the compression of the mass. The force exerted on the mass of ECCOSIL 4952 may be as high as pounds; however, where thethermoelectric unit 181 is stress-resistant, the force may be as low as 15 pounds. When the gap has the desired spacing the mass is secured bystraps 233 ofspring 235. The spring 235 (FIG. 5) is composed of sheet titanium alloy, typically about 0.005 inch in thickness. This spring has anannular center 251 from which thestraps 233 extend radially uniformly spaced around its periphery. ln securing theplug 231, thecenter 251 is spot welded to thesloping shoulder 253 of thehead 237 and thestraps 233 are bent around the head and spot welded to the thickenedportion 129 of theshell 127.

Initially the container 43 (FIG. 3 which is composed of commercially pure titanium is in two parts. One part has anopening 261 for theoutput conductor 263 from thetransformer 35. Initially, thebattery 23, the printedcircuit boards 25 and 27, block 29, containing the circuit components (integrated circuit and separate transistors not shown) potted in EPOXY resin, thestorage capacitor 31, themagnetic switch 33 and thetransformer 35 are assembled and connected outside of thecontainer 43.

Theoutput conductor 263 is also initially connected into a feed-through assembly 264 (FIG. 3). This assembly includes a ferrite radio-frequency filter 265 which suppresses electromagnetic disturbances and which encircles theconductor 263. The assembly also inlcudes a ceramicinsulating sleeve 267 throgh which theconductor 263 is sealed gas-tight. Thesleeve 267 is sealed into aflanged sleeve 269, typically of titanium. Theferrite trap 265 is secured to the inner side of theflanged sleeve 269 by a spring washer 271. Theconductor 263 is connected near its external end to aflexible connector 272.

Once the components are interconnected thebattery 23 and the parts 25-35 are mounted in one of the parts of thecontainer 43 with the battery cradled between theboards 25 and 27. Theboards 25 and 27 are contoured to rest on the inner surface of thecontainer 43. The ground conductors (FIG. 13) are connected to thecontainer 43. Theassembly 264 is disposed adjacent theopening 261 with theflanges 273 ofsleeve 269 appropriately positioned adjacent thehole 261. The two parts of thecontainer 43 are then welded to form the container. The ECCOSIL 4952mass 47 between the cold-junction end 49 of the battery and thecontainer 43 is injected with a syringe inserted throughopening 261. The 2CN potting rubber is then injected throughopening 261 and encompasses theparts 23 through 35. Thecontainer 43 and its content are then placed in a chamber (not shown) which is evacuated and filled with an inert gas (argon) at about one atmospheric pressure. The inert gas permeates theconverter 63 through theopening 231 and thebushing 243. Within the chamber, in the inert-gas atmosphere, theflange 273 is welded gas-tight to the rim of thehold 261 sealing the hole. Theconnector 272 is now connected to aterminal block 275.

Theterminal block 275 is in the form ofa rectangular parallelapiped having a cylindrical opening through which theinner end 277 of thecatheter 39 passes. Laterally a set screw 279 (FIG. 2) is provided in theblock 275. Over the head of theset screw 279 there is asilastic plug 281.

Thecatheter 39 is inserted inheart pacer 21 by the doctor who installs it in the hosts heart. During the construction of thepacer 21 thecatheter 39 is replaced by a pin (not shown) of the diameter of thecatheter 39. This pin is encircled by asuture boot 283. The pacer as now assembled is mounted in a mold (not shown) with thedimple 51 which forms the window facing downwardly and masked. TheEPOXY resin 50 is then molded about the titanium casing except at thedimple 51 and theEPOXY excapsulation 285 for theconductor 263, theconnector 272, theblock 275, the bottom 283 and the pin (not shown) is formed. The pin is then removed.

After installation in the heart ofthe host, thecatheter 39 is inserted in the opening formed by the pin and theend 277 is secured by theset screw 279 in the terminal block. The head of the set screw is closed by theplug 281 and theplug 281 can be sealed by silastic insulation.

The electrical circuit (FIG. 13) used in the practice of this invention is of the solid-state type and includes, in addition to themagnetic switch 33, a DC toDC converter 301, andamplifier 303, a monostable 305, a noise-rate turn-on 307, amultivibrator reset 309, amultivibrator 311, and anoutput 313.

The output of thebattery 23 is about 0.6 volt open circuit; the DC-DC converter 301 derives about 5.4 volts from this output for operating the remainder of the circuit.

Theamplifier 303 amplifies any input signal impressed on itsinput terminal 305 which is greater than 1.5 millivolts to 2.0 millivolts. This stage amplifies R- waves from the heart, pacer pulses and any noise which might be present at theinput 315. The gain of thisamplifier 303 falls off rapidly once the frequency of the incoming noise is out of the ampliflers band pass.

The monostable 305 performs two different functions. First, when it is triggered by a signal from theamplifier 303, it puts out typically a 0.275 second duration square pulse; and the monostable 305 cannot be triggered on again until it times out. This pulsing interval serves as the pacers refractory period. The pacers mode cannot be affected during this time. Theamplifier 303 ramains sensitive, but themultivibrator 311 cannot be reset until themonstable 305 completes its pulse of 0.275 second. Theamplifier 301 remains operative to drive the noise rate turn-on 307. Second, the leading edge of the monostable pulse resets themultivibrator 311.

Themultivibrator 311 controls the rate and width of the output pulses of thepacer 21. Themultivibrator 311 can operate in two different modes. One mode is the normal rate which is 702 beats per minute. The other is the noise rate which is beats per minute. In both modes the pulse duration remains at I.Ii'.0l milliseconds.

Theoutput 313 is the stage which produces the nega-v tive pulse which actually paces the heart.

The multivibrator reset 309 stops themultivibrator 311 from putting out a pulse (heart pacing pulse). If a monostable pulse (duration 0.275) occurs because of an R-wave fed back from the heart or a pacer pulse, thisstage 309 resets the multivibrator rate capacitor (not shown) to near zero volts. Every time themultivibrator 311 is reset it waits for 850 milliseconds (70.6 beats per minute) before it puts out another pacer pulse. Every time thepacer 21 itself puts out a heart pacing pulse it resets itself.

The noise-rate turn-on 307 constantly monitors the output of theamplifier 303. If the noise rate circuit senses that the amplifiers output pusles are at a specific frequency (15 hertz i5 hertz, the noise turn-one frequency) it disables themonostable stage 305. This in turn prevents themultivibrator reset stage 309 from discharging the rate capacitor (not shown) in themultivibrator 311. If the rate capacitor in the multivibrator is not discharged, which happens every time themonostable stage 305 puts out its 0.275 second pulse, themultivibrator 311 continues to run, but the capacitor does not discharge to zero on every pulse and the rate increases to 85:5 beats per minute (the noise rate). When themagnetic reed switch 33 is closed by a magnet, external of the body, thepacer 21 becomes a fixed rate unit at the noise rate.

In the heart pacer under the control of the circuit shown in FIG. 13 R-waves, when they occur, disable themultivibrator 311 from delivering heart-pacer pulses to the heart. Pulses are only delivered in the absence of R-waves. The operation of such a heart pacer is referred to as an R-wave-inhibited demand pacer.

Typical parameters of the heart pacer in accordance with this invention are:

1. Pulse Duration l.l milliseconds, nominal 1.0-1.2 milliseconds, range. A sufficiently long pulse duratino to always insure heart capture is provided. A minimum pulse duration is desired to conserve electrical power output. If the pulse duration were reduced substantially below the 1.1 milliseconds, the amplitude requirement may become excessive, thereby causing fibrillation.

2. Pulse Amplitude 8 milliamps, nominal 7.5 to

8.5 milliamps, range. This amplitude is selected to always provide effective capture after the rise of threshold levels after electrode endothelialization. A higher amplitude, possibly milliamps, may be considered, since a nuclear powered system is not battery energy sensitive, but the possibility of fibrillation might arise with such high levels. A lower amplitude in some are cases could prevent heart capture.

3. Basic Rate 70 beats per minute, nominal 68 to 72 beats per minute range. This rate appears to be the most common required by patients.

4. Noise Rate 85 beats per minute, nominal 80 to 100 beats per minute, range. This is the rate at which the pacemaker operates when noise interference is so great that it masks the normal O-R-S heart complex. A rate higher than the basic rate is selected to minimize the possibility of competition with the normal heart rate should it be in normal rhythm.

5. Noise Rate Turn-On Approximately Hertz. This rate is selected to prevent interference from all conceivable noise modes, microwave ovens and the like, with the exception of a sporadic impulse which would not be fatal. It is sufficiently low to rule out 60 Hertz rates which are the most probable, particularly with the expanding use of microwave ovens.

6. Magnetic Switch Rate 85 beats per minute, nominal 80 to 100 beats minute, range. This rate is a fixed rate, at which the pacer operates when energized by a magnet placed near the patients magnetic reed switch. This turnon feature allows checking of the pacemaker performance since in the R-wave inhibited mode with normal Q-R-S complexes, the physican cannot tell whether the pacemaker is operating.

7. R-Wave Sensitivity i 1.75 millivolts, nominal 1.5 to 2.0 millivolts, range. This sensitivity is selected to assure sensing of the R-wave, and is sufficiently high to prevent interference from abnormal P waves.

8.Refractory Period 275 milliseconds, nominal 250 to 300 milliseconds, range. This electronic refractory period begins with either a normal R-wave or the pacemaker stimulus. It is sufficiently long to prevent stimuli during the T-wave onset or decaythis period being the critical period for possible pacemaker induced fibrillation, i.e., no stimulus occurs during the T-wave of normal heart repolarization.

In installing the pacer according to this invention in the body of the host, the surgeon makes anincision 401 in the body of thehost 402 preferably in the chest above the heart. Thecatheter 39 is then passed through a vein to the heart muscle. The catheter is then inserted in thepacer 21 and the pacer is inserted in the opening with the insulatingcoating 50 in contact with themuscles 403, FIG. 14, or 404, FIG. 15, and the window ordimple 51 away from themuscles 303 and in engagement with a non-muscular part of the chest, either therib cage 405, FIG. 14, or the skin. 406, FIG. 15.

While a preferred embodiment of this invention has been disclosed herein, many modifications thereof are feasible. Thisinvention is not to be restricted except insofar as is necessitated by the spirit of the prior art.

We claim:

1. A heart pacer for a host including a primary source of heat energy, a converter connected to said source for converting said energy into electrical energy, means, connected to said converter, for deriving electrical pulses from said electrical energy, outputconductor means including a catheter, connected to said deriving means and to be connected to the heart of said host, for impressing said pulses on said heart, and a container for said source, converter and deriving means, the said container being composed of electrically conducting material and being electrically connected to said deriving means to serve as ground therefor, the outer region of said container being coated with electrically insulating material except over a relatively small limited area thereof on one side only, the conducting material of said limited area to be connected electrically to non-muscular body parts of said host connecting said ground to the body of said host.

2. The heart pacer ofclaim 1 wherein the limited area is in the form of a dimple extending outwardly from the remainder of the container.

3. The heart pacer of claim 2 wherein the insulating coating is flush with the outer surface of the dimple.

4. The heart pacer ofclaim 1 wherein the deriving means includes electrical circuit-component means for converting the electrical energy from the converter into pulses, and the output circuit-conductor means includes output assembly means, said output-assembly means including a feed-through assembly means, said output assembly being connected through the feedthrough assembly means to the deriving means within the container and extending out of the container and being connected outside of the container to the catheter.

5. The heart pacer of claim I wherein the container is of generally ellipsoidal form and the small limited area is a dimple of generally oval form.

Claims (5)

1. A heart pacer for a host including a primary source of heat energy, a converter connected to said source for converting said energy into electrical energy, means, connected to said converter, for deriving electrical pulses from said electrical energy, output-conductor means including a catheter, connected to said deriving means and to be connected to the heart of said host, for impressing said pulses on said heart, and a container for said source, converter and deriving means, the said container being composed of electrically conducting material and being electrically connected to said deriving means to serve as ground therefor, the outer region of said container being coated with electrically insulating material except over a relatively small limited area thereof on one side only, the conducting material of said limited area to be connected electrically to non-muscular body parts of said host connecting said ground to the body of said host.

2. The heart pacer of claim 1 wherein the limited area is in the form of a dimple extending outwardly from the remainder of the container.

3. The heart pacer of claim 2 wherein the insulating coating is flush with the outer surface of the dimple.

4. The heart pacer of claim 1 wherein the deriving means includes electrical circuit-component means for convertinG the electrical energy from the converter into pulses, and the output circuit-conductor means includes output assembly means, said output-assembly means including a feed-through assembly means, said output assembly being connected through the feed-through assembly means to the deriving means within the container and extending out of the container and being connected outside of the container to the catheter.

5. The heart pacer of claim 1 wherein the container is of generally ellipsoidal form and the small limited area is a dimple of generally oval form.

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